(a). Field of the Invention
The present invention relates to a charge coupled device having two-layer electrodes and, more particularly, to a charge coupled device of a two-phase driven/two-layer electrode type for use in a solid-state imaging device. The present invention also relates to a method for manufacturing such a charge coupled device.
(b). Description of the Related Art
Referring first to FIG. 1, there is shown a plan view of a general configuration of a solid-state imaging device. In FIG. 1, each of photoelectric transducer 301 arranged in a matrix converts incident light supplied thereto into a signal charge corresponding to the quantity of the incident light. The resultant signal charge is firstly read out to a vertical charge transfer section 302, and then transferred through a horizontal charge transfer section 303 so that output image signals are fed out from an output circuit 304.
FIGS. 2A to 2D show the steps of a manufacturing process for a first conventional charge coupled device of a buried channel/two-phase driven/two-layer electrode type, which is used in the horizontal charge transfer section of a solid-state imaging device of the type described above (see IEDM Technical Digest, 1973, pp. 24 and IEDM Technical Digest, 1975, pp. 55). The structure of the first conventional charge coupled device will be described by way of the steps of the manufacturing process.
Referring first to FIG. 2A, in a P-type (first conductivity type) semiconductor substrate 401, an N-type (second conductivity type) semiconductor layer 402 is formed by an ion implantation technique or the like. Then, N-type semiconductor layer 402 is subjected to a thermal oxidation to form a first insulating film 403 thereon. Then, a metallic pattern of first charge transfer electrodes 404a and 404b is formed on the first insulating film 403 by using a known sputtering and patterning technique. Subsequently, first insulating film 403 is selectively removed by using the pattern of first charge transfer electrodes 404a and 404b as a mask, followed by a second thermal oxidation to form a second insulating film 405 on the entire surface of the substrate to cover first charge transfer electrodes 404a and 404b and the N-type semiconductor layer 402.
Subsequently, P-type impurity ions (e.g., boron ions B.sup.+) are lightly implanted into regions of N-type semiconductor layer 402 located between each two of the first charge transfer electrodes 404a and 404b, thereby forming N.sup.- -diffused regions 406 in self-alignment with first charge transfer electrode 404a and 404b (see FIG. 2B).
Thereafter, a pattern of second charge transfer electrodes 407a and 407b is formed on the surface of the second insulating film 405 by using a known sputtering and patterning technique such that each of the second charge transfer transfer electrodes 407a and 407b covers a corresponding N.sup.- -diffused region 406 and the edge portions of first charge transfer electrodes 404a and 404b (see FIG. 2C). Subsequently, by utilizing a known technique, an interlayer insulating film (not shown) is formed and then a metallic wiring pattern 408 constituting a pair of signal lines for receiving two-phase driving signals is formed thereon.
Charge transfer electrodes 404a and 404b are connected to the pair of signal lines in such a way that each of the first charge transfer electrodes, e.g., 404a, and corresponding one second charge transfer electrodes, e.g., 407a, located adjacent to it are connected together to one of the signal line, thereby forming an electrode pair. If one numbers the plurality of pairs from one end, odd numberred pairs are connected to a first signal line to which a first signal .phi.1 is supplied, and even numberred pairs are connected to a second signal line to which a second signal .phi.2 is supplied, or vice versa. In this manner, the plurality of pairs of charge transfer electrodes are connected alternately to the first and second signal lines. Thus, the first conventional charge coupled device of a two-phase driven/two-layer electrode type is obtained (see FIG. 2D).
The operation of the first conventional charge coupled device will now be described with reference to FIGS. 3A to 3E. Referring first to FIG. 3A showing again the structure of the first charge coupled device, the charge coupled device having first charge transfer electrodes denoted by numeral 501a and 501b, second charge transfer electrodes denoted by numerals 502a and 502b, N-diffused regions denoted by numeral 503, N.sup.- -diffused regions denoted by numeral 504, and a P-type semiconductor substrate denoted by numeral 505.
The channel regions 503 and 504 underlying each odd numberred pair of first and second charge transfer electrodes 501a and 502a are connected to the first signal line .phi.1 and are designated by L1' and L2', respectively. The channel regions 503 and 504 underlying each even numberred pair of first and second charge transfer electrodes 501b and 502b connected to the second signal line .phi.2 are designated by L3' and L4', respectively. A channel region group L including channel regions L1' to L4' appears iteratively along the buried charge transfer channel.
FIGS. 3B to 3D show potential distributions at different time instants along the buried channel in the charge coupled device of FIG. 3A while showing each channel region in correspondence with the location thereof. FIG. 3E shows a timing chart for signal levels of two-phase driving signals .phi.1 and .phi.2, and FIGS. 3B, 3C and 3D correspond to time instants tb, tc and td in FIG. 3E, respectively.
At time instant tb, driving signals .phi.1 and .phi.2 are at an H-level V.sub.H and an L-level V.sub.L. respectively. At this time, assuming that P.sub.L1, P.sub.L2, P.sub.L3 and P.sub.L4 are electric potentials at respective channel regions L1 to L4, the potential distribution is such that the relationship P.sub.L1 &lt;P.sub.L2 &lt;P.sub.L3 &lt;P.sub.L4 holds, as shown in FIG. 3B. The signal charges 510A and 510B are stored in channel regions L1 underlying first charge transfer electrodes 501a of the first pairs to which an H-level voltage V.sub.H are now applied. At time instant tc subsequent to tb, the levels of driving signals .phi.1 and .phi.2 are shifted to become substantially equal to each other at the middle potential. Due to the shifts in the electric potentials at the charge transfer electrodes, the potential distribution of the channel regions is changed so that the relationship P.sub.L1 =P.sub.L3 &lt;P.sub.L2 =P.sub.L4 holds, as shown in FIG. 3C. At this time, the signal charges 510A and 510B remain in channel regions L1. At time instant td, the levels of signals .phi.1 and .phi.2 shift to an L-level V.sub.L and to an H-level V.sub.H, respectively, and the potential distribution of the channel regions is changed so that the relationship P.sub.L3 &lt;P.sub.L4 &lt;P.sub.L1 &lt;P.sub.L2 holds. As a result, the signal charges 510A and 510B are transferred towards channel regions L3' in which the deepest electric potentials are created by the overlying electrodes at which the H-level V.sub.H are now applied.
By iterating the above cycle, each of the signal charges 510A, 510B and 510C is transferred toward the left through channel regions L1' and L3'. Channel regions L2' and L4', in each of which a shallow electric potential is created by each of overlying second charge transfer electrodes 502a and 502b, are formed in order to prevent signal charges from flowing reversely and hence to restrict signal charges to be transferred only leftward in the drawings.
FIGS. 4A to 4E show the steps of a manufacturing process for a second conventional charge coupled device of a buried channel/two-phase driven/two-layer electrode type (see Patent Publication Nos. JP-A-61-184876 and JP-A-61-184877). Referring first to FIG. 4A, N-impurity ions are implanted into a P-type semiconductor substrate 601 to form an N-type semiconductor layer 602 therein, followed by a thermal oxidation to form a first insulating film 603 thereon.
Next, by utilizing a well known sputtering and patterning technique, a plurality of first charge transfer electrodes 604a and 604b are formed on the first insulating film 603 at a predetermined pitch. Subsequently, first insulating film 603 is selectively removed by using the pattern of first charge transfer electrodes 604a and 604b as a mask, followed by a second thermal oxidation to form a second insulating film 605 covering the entire surface including surfaces of first charge transfer electrodes 604a and 604b and the spaces therebetween.
Subsequently, as shown in FIG. 4B, P-type impurity ions (e.g., boron ions B.sup.+) are lightly implanted into regions of N-type semiconductor layer 602 between each two of first charge transfer electrodes 604a and 604b. As a result, N.sup.- -diffused regions 606 are formed in self-alignment with first charge transfer electrodes 604a and 604b.
Thereafter, as shown in FIG. 4C, a photoresist pattern 610 is formed so as to cover an area extending from the central position, as viewed along the channel, of each of first charge transfer electrodes 604a and 604b to the central position of one of the N.sup.- -diffused regions 606 adjacent to the each of first charge transfer electrodes. Then, P-type impurity ions (e.g., boron ions B.sup.-) are lightly implanted into a part of each N.sup.- -diffused region 606 by using photoresist pattern 610 as a mask to form N.sup.-- -diffused regions 607 each having an edge self-aligned with the edge of each of first charge transfer electrodes 604a and 604b.
Subsequently, second charge transfer electrodes 608a and 608b are formed each overlying both N.sup.- -diffused regions 606 and N.sup.-- -diffused regions 607, by a well known sputtering and patterning technique, such that the edge portions of second charge transfer electrodes 608a and 608b overlap with the edge portions of first charge transfer electrodes 604a and 604b (see FIG. 4D). As a result, a charge transfer electrode scheme is obtained in which the first electrodes and the second electrodes are alternately arranged one by one along the buried channel.
Thereafter, an interlayer insulating film (not shown) is formed by utilizing a known sputtering and patterning technique, followed by forming a metallic wiring pattern 609 constituting a pair of signal lines to which the two-phase driving signals .phi.1 and .phi.2 are applied. The charge transfer electrodes are connected such that each first charge transfer electrode 604a or 604b and a corresponding one of second charge transfer electrode 608a or 608b adjacent to each other are connected together to form a pair. Each pair is connected to the metallic wiring pattern 609 such that each odd numberred pair are connected to a first signal line (.phi.1), and each even numberred pair are connected to a second signal line (.phi.2). Thus, the second conventional charge coupled device of a two-phase driven/two-layer electrode type is obtained, as schematically shown in FIG. 4E.
The operation of the second conventional charge coupled device will now be described with reference to FIGS. 5A to 5E. FIGS. 5A to 5E show, similarly to FIGS. 3A to 3E, the structure and the operation of the second conventional charge coupled device. Referring first to FIG. 5A, the charge coupled device has a plurality of first charge transfer electrodes denoted by 701a and 701b, second charge transfer electrodes denoted by 702a and 702b, N-diffused regions denoted by 703, N.sup.- -diffused regions denoted by 704, N.sup.-- -diffused regions denoted by 705, and a P-type silicon substrate denoted by 706. The charge transfer channel formed of three regions having different impurity concentrations, i.e., N-diffused regions, N.sup.- -diffused regions and N.sup.-- -diffused regions provides an iterative potential distribution including six potential levels P.sub.L1 to P.sub.L6 at respective channel regions L1 to L6 forming a single channel group iteratively appearing along the buried channel.
At time instant tb, driving signals .phi.1 and .phi.2 are shifted to an H-level V.sub.H and at an L-level V.sub.L, respectively. At this time, the electric potentials of channel regions L1 to L6 are such that the relationship P.sub.L1 &lt;P.sub.L2 &lt;P.sub.L3 &lt;P.sub.L4 &lt;P.sub.L5 &lt;P.sub.L6 holds. Each of signal charges 710A and 710B is stored in a channel region L1 underlying the electrode to which the H-level is applied and therefore having the deepest electric potential among one channel group L.
At time instant tc, driving signals .phi.1 and .phi.2 have an electric potential corresponding to a middle level between the H-level V.sub.H and the L-level V.sub.L, and the electric potentials of channel regions L1 to L6 shift such that the relationship P.sub.L1 =P.sub.L4 &lt;P.sub.L2 =P.sub.L5 &lt;P.sub.L3 =P.sub.L6 holds. At this time, the signal charges remain in channel regions L1. Subsequently, at time instant td, during which driving signals .phi.1 and .phi.2 are at a L-level V.sub.L and a H-level V.sub.H, respectively, the electric potentials of channel regions L1 to L6 shift such that the relationship P.sub.L4 &lt;P.sub.L5 &lt;P.sub.L6 &lt;P.sub.L1 &lt;P.sub.L2 &lt;P.sub.L3 holds.
Each of the signal charges 710A, 710B and 710C is transferred leftward in the drawing towards a corresponding one of channel regions L4, which is now maintained at the deepest electric potential due to the potential distribution. Thereafter, by iterating these cycles, each of the signal charges 710A, 710B and 710C is transferred leftward in the drawing along the buried channel. Channel regions each having a shallow electric potential and underlying second charge transfer electrode 702a or 702b are provided in order to prevent signal charges from flowing in the reverse direction and hence to restrict signal charges to be transferred only to the left as viewed in the drawings.
In general, the time period required for transfer of signal charges along a charge coupled device increases exponentially with an increase in the gate length of each charge transfer electrode. More specifically, if the first conventional charge coupled device of a two-phase driven/two-layer electrode type is used as a horizontal transfer section of a solid-state imaging device such as shown in FIG. 1, the following problem occurs. In an imaging device having a long pixel pitch (L) in the horizontal direction, the large length of each charge transfer electrode, which depends on the pixel pitch L, requires a large time period for transfer of signal charges, resulting in a poor transfer efficiency.
The second conventional charge coupled device of a two-phase driven/two-layer electrode type was proposed so as to improve the poor transfer efficiency of the first conventional charge coupled device. In the second conventional device, six charge transfer electrodes provided within each horizontal pixel pitch L presents a smaller length of charge transfer electrode, which is about 2/3 of that of the first conventional charge coupled device having four charge transfer electrodes, so that the effective charge transfer speed is increased up to about 3/2 times that of the first conventional device.
In the technical field of solid-state imaging devices, the demand for greater charge transfer speed is increasing more and more. For example, in a horizontal charge transfer section such as shown in FIG. 7 of a solid-state imaging device having two horizontal charge transfer sections as shown in FIG. 6, the effective length of each charge transfer electrode is about 2 times that of the solid-state imaging device having a single horizontal charge transfer section shown in FIG. 1. Even if the scheme of second conventional charge coupled device is applied to such a solid-state imaging device having two horizontal charge transfer sections, the charge transfer speed cannot be improved to a sufficient level. Therefore, charge coupled devices having further improved transfer efficiency are desired.
Further, in the second conventional charge coupled device, since three diffused regions having different impurity concentrations are formed, a larger number of manufacturing steps are required compared to the first conventional charge coupled device. This also increases manufacturing costs.